Limb Chondrogenesis of the Seepage Salamander, Desmognathus Aeneus (Amphibia: Plethodontidae)
JOURNAL OF MORPHOLOGY 265:87–101 (2005) Limb Chondrogenesis of the Seepage Salamander, Desmognathus aeneus (Amphibia: Plethodontidae) R. Adam Franssen,1 Sharyn Marks,2 David Wake,3 and Neil Shubin1* 1University of Chicago, Department of Organismal Biology and Anatomy, Chicago, Illinois 60637 2Humboldt State University, Department of Biological Sciences, Arcata, California 95521 3University of California, Museum of Vertebrate Zoology, Department of Integrative Biology, Berkeley, California 94720 ABSTRACT Salamanders are infrequently mentioned in referred to two amniote model systems (Gallus, analyses of tetrapod limb formation, as their development chicken; Mus, mouse), on which most research has varies considerably from that of amniotes. However, focused. However, the differences between urodeles urodeles provide an opportunity to study how limb ontog- and amniotes are so great that it has been proposed eny varies with major differences in life history. Here we assess limb development in Desmognathus aeneus,a that the vertebrate limb may have evolved twice direct-developing salamander, and compare it to patterns (Holmgren, 1933; Jarvik, 1965). Further, the differ- seen in salamanders with larval stages (e.g., Ambystoma ences between urodeles and anurans are significant mexicanum). Both modes of development result in a limb enough to have suggested polyphyly within amphib- that is morphologically indistinct from an amniote limb. ians (Holmgren, 1933; Nieuwkoop and Sutasurya, Developmental series of A. mexicanum and D. aeneus were 1976; Jarvik, 1965; reviewed by Hanken, 1986). Re- investigated using Type II collagen immunochemistry, Al- searchers have studied salamander morphology cian Blue staining, and whole-mount TUNEL staining. In (e.g., Schmalhausen, 1915; Holmgren, 1933, 1939, A. mexicanum, as each digit bud extends from the limb 1942; Hinchliffe and Griffiths, 1986), described the palette Type II collagen and proteoglycan secretion occur patterns of amphibian limb development with refer- almost simultaneously with mesenchyme condensation.
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